1. Field of the Invention
The present invention relates to a multi-displacement detecting apparatus capable of simultaneously detecting a plurality of displacements including a rotary displacement and a radial displacement of a spindle of a machine tool.
2. Description of the Related Art
A conventional multi-displacement detecting apparatus designed for the spindle of a machine tool using a magnetic bearing and having a built-in motor requires multiple sensors such as a rotary displacement sensor for controlling a rotational speed or a position of the motor and a radial displacement sensor for assisting the magnetic bearing in controlling the radial position of the spindle.
FIG. 1 is an oblique view of a modeled example of the spindle using a magnetic bearing with a conventional multi-displacement detecting apparatus incorporated therewith and having a built-in motor. A shaft (spindle) 1 is provided with a motor 2, a magnetic thrust bearing 3, magnetic radial bearings 4 and 5, and a resolver 6 for detecting a rotary displacement of the rotor and thus controlling the rotation of the motor 2. Then, thrust displacement sensors 7 and 8 for controlling the magnetic thrust bearing 3 are mounted on the ends of the shaft 1. Radial displacement sensors 9 and 10 for controlling the magnetic radial bearing 5, and radial displacement sensors 11, 12, 13 and 14 for controlling the magnetic radial bearing 4 are mounted on the side of the shaft 1.
FIG. 2 shows an example of the structures of the radial displacement sensors 11, 12, 13 and 14 shown in FIG. 1. FIG. 3 shows an example of the structure of the resolver 6 shown in FIG. 1. FIG. 4 is a block diagram showing an example of a conventional multi-displacement detecting apparatus made up of the radial displacement sensors, the resolver, and a detecting circuit that uses multiple arithmetic logic units to convert the detected signals sent from the sensors and resolver into quantities of displacement. C-shaped cores 21, 22, 23 and 24 each having two teeth and made of silicon steel or another magnetic substance are arranged around a shaft 1, to which a silicon steel or other magnetic substance-made cylinder 15 is attached, at intervals of 90.degree. so that the teeth will face the center of the shaft 1. Each of primary coils 31, 32, 33 and 34 is respectively wound around one of the teeth of each of the C-shaped cores 21, 22, 23 and 24. The primary coils 31, 32, 33 and 34 are connected in series and excited with an AC signal EX from an oscillator 29. Each of secondary coils 25, 26, 27 and 28 is respectively wound around the other tooth of each of the C-shaped cores 21, 22, 23 and 24. Of the secondary coils 25 to 28, mutually-opposing coils 25 and 27, and 26 and 28 are connected in series respectively so that the output signals their mates provide will be 180.degree. out of phase.
In each of the radial displacement sensors 11, 12, 13 and 14 constituted as mentioned above, the reluctance varies with the variations in air gaps between the C-shaped cores 21, 22, 23 and 24 and the magnetic substance cylinder 15 that are attributable to a displacement in the position of the shaft 1. The variation in reluctance causes the secondary coils 25, 26, 27 and 28 to provide the output signals Xp, Yp, Xn and Yn. The output signals Xp, Yp, Xn and Yn are determined by X- and Y-axis displacements dX and dY, and represented as expressions (1) to (4). EQU Xp=(K+dX).multidot.EX (1) EQU Yp=(K+dY).multidot.EX (2) EQU Xn=(K-dX).multidot.EX (3) EQU Yn=(K-dY).multidot.EX (4)
where, K is a constant coefficient.
The output signals Xp and Xn or Yp and Yn are reverse in polarity and connected in series. Then, subtractors 18 and 19 perform subtractions of the below expressions (5) and (6) on the output signals. EQU XE=Xp-Xn=2dX.multidot.EX (5) EQU YE=Yp-Yn=2dY.multidot.EX (6)
These two output signals XE and YE are sampled and held by an SMPL signal from the oscillator 29 by sample-and-hold circuits 35 and 36, when an AC signal EX shows a maximum gain. Assuming that the maximum gain of the AC signal EX is "1/2", the output signals Xo and Yo of the sample-and-hold circuits 35 and 36 are given by assigning "EX=1/2" to the expressions (5) and (6) or represented as the below expressions (7) and (8). EQU Xo=dX (7) EQU Yo=dY (8)
Thus, the displacements in the X-axis and Y-axis directions of the shaft 1 can be detected.
A ring-type stator core 17 made of a silicon copper plate or other magnetic substance plate has eight teeth 41, 45, 42, 46, 43, 47, 44 and 48 arranged at intervals of 45.degree. on its inner circumferential surface. Primary coils 51, 55, 52, 56, 53, 57, 54 and 58, which are wound around the teeth 41, 45, 42, 46, 43, 47, 44 and 48 of the stator core 17 to face different directions, are connected in series and excited with the AC signal EX from the oscillator 29. Secondary coils 61, 65, 62, 66, 63, 67, 64 and 68 are wound around the teeth 41, 45, 42, 46, 43, 47, 44 and 48 of the stator core 17. Of the secondary coils, four alternate coils 61, 62, 63 and 64, or 65, 66, 67 and 68 are connected in series so that they will provide 180.degree. out-of-phase output signals alternately. A cylindrical resolver rotor 16 made of the silicon steel or another magnetic substance is attached to the shaft 1, which is waxing or waning at two points in comparison with a circle indicated by a dashed line D.
In the resolver 6 having the foregoing configuration, the reluctance varies with the displacement in the position of the shaft 1 and the variations in air gaps between the teeth 41, 45, 42, 46, 43, 47, 44 and 48 of the stator core 17 and the resolver rotor 16. The variation in reluctance causes the secondary coils 61, 65, 62, 66, 63, 67, 64 and 68 to provide output signals CXp, SAp, CYp, SBp, CXn, SAn, CYn and SBn. These output signals are determined by the rotary displacement of the shaft 1 and the X- and Y-axis displacements dX and dY, and represented as expressions (9) to (16). That is to say, the output signals CXp, SAp, CYp, SBp, CXn, SAn, CYn and SBn contain components whose levels are proportional to a sine-wave value of a rotary displacement, and have amplitudes modulated according to the variations in the gaps between the teeth and the resolver rotor 16. EQU CXp=(L+dX).multidot.(M+COS (2.multidot.(.theta.+dY/R))).multidot.EX(9) EQU SAp=(L+dA).multidot.(M+SIN (2.multidot.(.theta.+dB/R))).multidot.EX(10) EQU CYp=(L+dY).multidot.(M-COS (2.multidot.(.theta.-dX/R))).multidot.EX(11) EQU SBp=(L+dB).multidot.(M-SIN (2.multidot.(.theta.-dA/R))).multidot.EX(12) EQU CXn=(L-dX).multidot.(M+COS (2.multidot.(.theta.-dY/R))).multidot.EX(13) EQU SAn=(L-dA).multidot.(M+SIN (2.multidot.(.theta.-dB/R))).multidot.EX(14) EQU CYn=(L-dY).multidot.(M-COS (2.multidot.(.theta.+dX/R))).multidot.EX(15) EQU SBn=(L-dB).multidot.(M-SIN (2.multidot.(.theta.+dA/R))).multidot.EX(16)
where, L and M are constant coefficients, and R is a mean radius of the resolver rotor 16. A numeral dA represents an expression (17), and dB an expression (18). ##EQU1##
In each of the expressions 9 to 16, a quotient of dZ/R (Z=Y, A, X or B) in a sine (SIN) or cosine (COS) is negligible. Therefore, a composite output signal Co or So of output signals of the four secondary coils connected in series is represented as an expression (19) or (20). EQU Co=CXp-CYp+CXn-CYn=4L.multidot.COS (2.theta.).multidot.EX (19) EQU So=SAp-SBp+SAn-SBn=4L.multidot.SIN (2.theta.).multidot.EX (20)
These two output signals Co and So are sampled and held by the SMPL signal from the oscillator 29 by the sample-and-hold circuits 37 and 38, when the AC signal EX shows the maximum gain. Assuming that the maximum gain of the AC signal EX is "1/2", output signals C and S of the sample-and-hold circuits 37 and 38 are given by assigning "EX=1/2" to the expressions (19) and (20) or represented as the below expressions (21) and (22). EQU c=2L.multidot.COS (2.theta.) (21) EQU S=2L.multidot.SIN (2.theta.) (22)
Then, the output signals C and S whose levels are proportional to a cosine-wave value and a sine-wave value of a two-fold value of a rotary displacement .theta. of the shaft 1 are input in an inverse tangent arithmetic logic unit 30. This provides a signal .theta.o=2.times..theta.. Thus, a value can be detected in direct proportion to a rotary displacement of the shaft 1.
In the aforesaid conventional multi-displacement detecting apparatus, multiple displacement sensors are arranged in the axial direction of a main shaft. This extends the axial length, complicates the mounting of the displacement sensors, and increases cost.